Canine hypothyroidism is the most common endocrine problem in dogs. It is characterized by insufficient levels of thyroid hormones (T3/T4). Metabolic consequences include weight gain, lethargy, intolerance to cold, a slow heart rate, absence of heat cycles, and lethargy. The dogs typically exhibit hair loss and brittle coat, and skin which is dry, thick, puffy, and darkly pigmented. Some dogs develop secondary seborrhea. Estimates of incidence range from 1 in 156 to 1 in 500 animals. The onset of disease is typically in middle age (4-6 years). Current treatment is typically with thyroid hormone supplementation, generally with synthetic L-thyroxine (L-T4). The prognosis for dogs that respond to therapy is good; however this is a progressive disease, and the dogs should be monitored regularly and the medication dose adjusted as needed. There are two histological classifications of primary hypothyroidism. The first, lymphocytic thyroiditis, is characterized by lymphocytic infiltration of the thyroid tissue, and the second, idiopathic atrophy, is thought to be the end stage to lymphocytic thyroiditis where the thyroid tissue has been mostly destroyed. Canine hypothyroidism is believed to be an autoimmune condition, and antibodies against thyroid hormone and thyroglobulin are often found in the serum of affected dogs. There is also a breed predisposition for hypothyroidism, indicating that there is some heritable component to the disease and that genetic association studies may give some insight into the disease.
Accurate diagnosis of hypothyroidism presents challenges. While a normal level of free T4 generally indicates that the dog does not have hypothyroidism, a low-normal or below-normal level in a single test does not necessarily mean the dog is hypothyroid. A more accurate diagnosis can be obtained by a full thyroid panel test that evaluates both free and total T3 and T4 as well as auto-antibodies against the thyroid hormones. Assays for thyroglobulin autoantibodies may also be used, but these autoantibodies are present at detectable levels only in about 50 percent of dogs with autoimmune thyroiditis. Although certain organizations maintain a hypothyroid registry to help identify whether dogs are suitable for breeding, no specific genetic markers for canine hypothyroidism have been disclosed. Diagnosis often comes after the dogs have already been bred. Hypothyroidism is a progressive autoimmune disease so there is an advantage to diagnosing and treating an animal early in the onset of disease in addition to identifying the animal as a potential carrier for the condition. There is a need for improved methods of identifying and treating dogs at elevated risk of this disease.
Other autoimmune diseases or conditions of dogs include lymphocytic thyroiditis and idiopathic atrophy of the thyroid, rheumatoid arthritis, systemic lupus erythematosus (SLE), discoed lupus, auto-immune hemolytic anemia (AIHA), or pemphigus. As in the case of hypothyroid disease, there is a need for improved methods of identifying and treating dogs at elevated risk of such diseases, particularly as early treatment, before there is irreversible tissue or organ damage, is critically important.